1. Field of the Invention
The present invention is concerned with a three-way catalyst system useful for the treatment of exhaust gases such as automobile engine exhaust gases, and more particularly such a system which is effective for suppressing the formation of hydrogen sulfide.
2. Description of Related Art
The use of three-way catalysts to treat internal combustion engine, e.g., automobile engine, exhaust gases by substantially simultaneously treating unburned hydrocarbons, carbon monoxide and nitrogen oxides in the gases is well known. However, in many cases, these same catalysts convert sulfur compounds contained in the exhaust into hydrogen sulfide, H.sub.2 S, noted for its extremely disagreeable "rotten egg" odor. Without intending to be bound by any particular theory, it is believed that the presence of appreciable amounts of ceria in the catalyst is the cause of H.sub.2 S generation when operating conditions go from fuel-lean to fuel-rich. Methods of suppressing formation of H.sub.2 S by including cobalt, nickel, iron, manganese or rhenium in the catalytic composition itself are known. For example, U.S. Pat. No. 4,552,733 provides a three-way catalyst comprising one or more platinum group metals dispersed on a gamma alumina support, the catalyst including a base metal oxide dispersed with the platinum group metal. The base metal oxide may be nickel, iron or manganese. This catalyst is stated to have three-way capability without producing appreciable quantities of hydrogen sulfide, sulfur trioxide or sulfuric acid. However, there is some opinion, particularly in Europe, that cobalt and nickel present potential health problems, so automotive manufacturers are increasingly reluctant to use catalysts containing these metals. In other cases, the net effect of including iron, manganese or rhenium in the catalyst is considered negative by manufacturers even though significant positive effects are obtained by their inclusion. While it is known to disperse copper with catalyst materials providing three-way activity, (see, for example, U.S. Pat. Nos. 4,675,308; 4,492,770; 3,993,572; and 4,492,769), there is a potential for copper oxide to reduce the activity of a three-way catalyst, if dispersed therewith.